I finally got around to making a copy of Pentagrid's fixture, which was
detailed in the thread " Fixture for boring soft jaws on a lathe"
posted in early May 2009.

Pentagrid's fixture consists of three pieces of spring steel 0.5" wide
by 0.080" (2 mm) thick by about 6". held in grooves milled in the sides
to the three soft jaws. The strips form an equilateral triangle, at
least initially.

My implementation uses scale-free spring steel 0.5" wide by 0.093" thick
by 6.312", long resting in milled grooves ~0.368" wide and 0.160" deep.
The ends of the spring steel strips are cut square to the length of the
trip, and roughly rounded in the thickness so they will bottom in the
corner of the milled grooves and not hang up on tool marks on the walls.

The grooves are parallel to the chuck rotation axis, and there are two
grooves per jaw, one on each side.

The 2mm (0.080") thickness isn't common in the US, so I used 0.094"
nominal (2.4 mm).

The spring steel is # 03342185 from MSC:
http://www1.mscdirect.com/CGI/NNSRIT?PMAKA=03342185.

It all worked quite well, but it took a LOT of force to tighten the
chuck. I had to use a cheater pipe on the chuck key. The spring steel
strips now have the desired bend in the center, so they now won't tend
to bow inward. I suppose there is no harm in using the cheater if not
done too often, and the next time may be easier now that the springs are
broken in A thinner spring may also be adequate, and require less
force..


Joe Gwinn